Piezoelectric device and method for manufacturing piezoelectric device

ABSTRACT

A piezoelectric device includes a piezoelectric thin film formed by separating and forming a piezoelectric single crystal substrate, an inorganic layer formed on a back surface of the piezoelectric thin film, an elastic body layer disposed on a surface opposite to the piezoelectric thin film of the inorganic layer, and a support pasted to a surface opposite to the inorganic layer of the elastic body layer. In a membrane structure portion, the inorganic layer and the elastic body layer are disposed on the piezoelectric thin film through a gap layer. The elastic body layer reduces a stress caused by pasting the piezoelectric thin film including the inorganic layer and the support and has a certain elastic modulus. The inorganic layer is formed with a material having an elastic modulus higher than that of the elastic body layer and suppresses damping caused by disposing the elastic body layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric device including apiezoelectric single crystal thin film, and more specifically apiezoelectric device including a membrane structure, and a method formanufacturing the piezoelectric device.

2. Description of the Related Art

Currently, a large number of piezoelectric devices in which apiezoelectric single crystal body is formed in a thin film have beendeveloped. In the piezoelectric devices containing such a piezoelectricthin film, a support member for supporting the piezoelectric thin filmis required in practical use. Such a support member is disposed on oneprincipal surface of the piezoelectric thin film as described inJapanese Unexamined Patent Application Publication No. 2007-228319 orJapanese Unexamined Patent Application Publication No. 2003-17967.

Some of the piezoelectric devices have a membrane structure in which, aspace is formed between a region in which an electrode which functionsas the piezoelectric device in the piezoelectric thin film is formed andthe support member, in order to increase the characteristics of thedevices.

Heretofore, a smart cut method has been used as one of the methods forforming a composite piezoelectric substrate containing a piezoelectricthin film and a support member. According to the smart cut method, anion implanted layer is formed by implanting ions into one principalsurface of a piezoelectric substrate having a joinable thickness. Next,the support member which is separately formed is joined to the principalsurface at the ion implanted layer side of the piezoelectric substrateon which the ion implanted layer is formed by using an activationjunction, affinity junction, or the like. Thereafter, the piezoelectricthin film is separated by heating from the piezoelectric substrateutilizing the ion implanted layer.

Therefore, when forming the piezoelectric device having the membranestructure, a sacrificial layer which serves as a gap layer later isformed on one surface of the piezoelectric substrate, and then thesupport member is joined to the front surface of the piezoelectricsubstrate on which the sacrificial layer is formed. Thereafter, thepiezoelectric thin film is separated from the piezoelectric substrate tobe formed, etching windows are formed in the piezoelectric thin film,and then the sacrificial layer is removed from the etching windows. Inthis case, a structure in which the piezoelectric thin film and thesupport member are joined to each other at portions other than themembrane structure portion is formed.

Mentioned as a method of joining the substrate for separating andforming a thin film and the support member is a method including placingan elastic body between a single crystal silicon substrate, which is nota composite piezoelectric substrate, and a support as described inJapanese Unexamined Patent Application Publication No. 2008-118079.

However, when directly joining a piezoelectric substrate and a supportmember as described in Japanese Unexamined Patent ApplicationPublication No. 2007-228319 or Japanese Unexamined Patent ApplicationPublication No. 2003-17967, the coefficient of linear expansiondifference between the piezoelectric substrate and the support membercannot be disregarded, and therefore materials of the support member areconsiderably limited. Moreover, in order not to apply unnecessary stressto the piezoelectric substrate during the junction process, it isrequired to set strict joining conditions in such a manner thatirregularities exceeding a certain level are not present or particles(foreign substances, dust, or the like of a minimum size) are notpresent on a joined surface, which increases the process load and makesthe process control difficult.

In particular, when forming the membrane structure, not only theflatness degree of the piezoelectric substrate surface but the flatnessdegree of the sacrificial layer surface needs to increase. Thesacrificial layer surface and the piezoelectric substrate surface cannotform the same plane in terms of a manufacturing process. Therefore, inorder to increase the flatness degree thereof to a certain level orhigher, the process load increases, which leads to an increase in cost.

In contrast, as described in Japanese Unexamined Patent ApplicationPublication No. 2008-118079, various problems occurring when joining thepiezoelectric substrate and the support member as described above arelessened by inserting an elastic body between the semiconductorsubstrate and the support member. However, by joining an elastic bodywith a low elastic coefficient to the piezoelectric substrate, dampingoccurs, so that the function as the piezoelectric device decreases. Thisphenomenon arises without exception even in the case of the membranestructure because at least a portion where the piezoelectric thin filmand the elastic body are joined to each other is present. Particularlyin a piezoelectric device utilizing elastic waves, the influence isconsiderable.

SUMMARY OF THE INVENTION

Therefore, preferred embodiments of the present invention provide apiezoelectric device including a membrane structure in which theoccurrence of the above-described various problems during the junctionprocess are prevented and in which the function does not decrease interms of the structure, and also provide a method for manufacturing thepiezoelectric device.

According to a preferred embodiment of the present invention, apiezoelectric device includes a piezoelectric thin film on which a driveelectrode is provided and a support member disposed on one principalsurface side of the piezoelectric thin film and including a membranestructure in which a gap layer is provided between a region in which thedrive electrode is provided on the piezoelectric thin film and thesupport member. Between the piezoelectric thin film and the supportmember of the piezoelectric device, an elastic body layer and aninorganic layer which is located at the piezoelectric thin film side ofthe elastic body layer and includes a material with a higher elasticmodulus and a higher hardness as compared with the elastic body layer,are provided.

In this configuration, since a composite layer including thepiezoelectric thin film and the inorganic layer is joined to the supportmember through the elastic body layer, the material of the supportmember having a larger thickness than the thickness of the piezoelectricthin film and the inorganic layer can be selected without considering adifference of the coefficient of linear expansion from that of thepiezoelectric thin film. Due to the fact that the inorganic layer isinserted between the elastic body layer and the piezoelectric thin film,damping caused by the elastic body layer does not occur.

The elastic body layer of a piezoelectric device according to apreferred embodiment of the present invention preferably includes aninorganic filler. In this configuration, due to the fact that aninorganic filler is compounded in the elastic body layer, the thermalconductivity of the elastic body layer can be increased and thecoefficient of linear expansion can be reduced and also the elasticmodulus can also be increased. Thus, various characteristics, such asthe power durability and the temperature characteristics of thepiezoelectric device, particularly a device utilizing acoustic waves,can be improved.

For the inorganic layer of a piezoelectric device according to apreferred embodiment of the present invention, a material having athermal conductivity higher than that of the piezoelectric thin film ispreferably used. In this configuration, since heat generated in thepiezoelectric thin film is effectively transmitted to the inorganiclayer, the power durability of acoustic waves device can be increased.

For the elastic body layer of a piezoelectric device according to apreferred embodiment of the present invention, a material having athermal conductivity higher than that of the piezoelectric thin film andthe inorganic layer is preferably used. In this configuration, sinceheat is effectively transmitted from the piezoelectric thin film to theelastic body layer through the inorganic layer, the power durability canbe further increased.

For the inorganic layer of a piezoelectric device according to apreferred embodiment of the present invention, a material having acoefficient of linear expansion lower than that of the piezoelectricthin film is preferably used. In this configuration, since the inorganiclayer is more difficult to deform than the piezoelectric thin film, theinorganic layer can constrain the piezoelectric thin film, and thetemperature characteristics as a piezoelectric device can be increased.

The piezoelectric thin film of a piezoelectric device according to apreferred embodiment of the present invention preferably includes amaterial made of a Group 1 element. In this configuration, when thematerial made of a Group 1 element, such as LT, LN, and LBO, is used forthe piezoelectric thin film, the position of the Group 1 element in thecrystal is displaced due to ion implantation, so that internal stress isaccumulated to cause cleavage in the thin film, and therefore crackingis likely to occur. However, by inserting the elastic body layer and theinorganic layer between the support member and the piezoelectric thinfilm, the support member is indirectly joined to the piezoelectric thinfilm, so that the defective fraction due to cracking during junction canbe particularly reduced.

According to another preferred embodiment of the present invention, amethod for manufacturing a piezoelectric device includes an ionimplantation process for implanting ionized elements into apiezoelectric substrate to thereby form a portion in which aconcentration of the elements implanted into the piezoelectric substratereaches a peak in the piezoelectric substrate, a sacrificial layerformation process for forming a sacrificial layer on a surface at theion implanted side of the piezoelectric substrate, an inorganic layerformation process for directly forming an inorganic layer on a surfaceat the ion implanted side of the piezoelectric substrate on which thesacrificial layer is formed, an elastic body layer disposing process fordisposing an elastic body layer on a surface opposite to thepiezoelectric substrate of the inorganic layer, a pasting process forpasting a support member to the elastic body layer, a peeling processfor peeling and forming the piezoelectric thin film from thepiezoelectric substrate in which the portion in which the concentrationof the elements implanted into the piezoelectric substrate reaches thepeak is formed, and a sacrificial layer removal process for removing thesacrificial layer to thereby form a gap layer.

According to the manufacturing method, the above-described piezoelectricdevice including a piezoelectric thin film, an inorganic layer, anelastic body layer, and a support member and including the membranestructure can be easily manufactured. In this case, by directly formingon the piezoelectric substrate with the sacrificial layer without usinga junction of the inorganic layer, and further disposing the elasticbody layer, problems occurring when joining the piezoelectric substrateand the support member are prevented. When the piezoelectric substrateincluding the sacrificial layer, the composite layer including theinorganic layer and the elastic body layer, and the support member, theinorganic layer serves as a protective layer of the piezoelectricsubstrate with the sacrificial layer and the elastic body layer servesas a buffer layer and a level difference reduction layer, problemsoccurring during a junction process or conventional problems describedabove do not occur. Thus, a piezoelectric device with high reliabilityand good characteristics can be manufactured with ease and with a highyield.

According to a further preferred embodiment of the present invention, amethod for manufacturing a piezoelectric device includes an ionimplantation process for implanting ionized elements into apiezoelectric substrate to thereby form a portion in which aconcentration of the elements implanted into the piezoelectric substratereaches a peak in the piezoelectric substrate, a sacrificial layerformation process for forming a sacrificial layer on a surface at theion implanted side of the piezoelectric substrate, an inorganic layerformation process for directly forming an inorganic layer on a surfaceat the ion implanted side of the piezoelectric substrate on which thesacrificial layer is formed, an elastic body layer disposing process fordisposing an elastic body layer on a front surface of the supportmember, a pasting process for pasting the inorganic layer and theelastic body layer, a peeling process for peeling and forming thepiezoelectric thin film from the piezoelectric substrate in which theportion in which the concentration of the elements implanted into thepiezoelectric substrate reaches the peak is formed, and a sacrificiallayer removal process for removing the sacrificial layer to thereby forma gap layer.

According to the manufacturing method described in the precedingparagraph, the elastic body layer is formed at the support member side,and then a first composite layer containing the piezoelectric substratewith the sacrificial layer and the inorganic layer and a secondcomposite layer containing the support member and the elastic body layerare pasted to each other in contrast to the above-describedmanufacturing method in which the piezoelectric substrate with thesacrificial layer is formed and the elastic body layer is formed at theinorganic layer side, and then the elastic body layer and the supportmember are pasted to each other. By providing such a process, theelastic body layer before pasting can be subjected to heat treatmentwith a heat temperature higher than the peeling temperature of theportion where the concentration of the elements implanted into thepiezoelectric substrate reaches the peak. Thus, annealing (removal of anunnecessary solvent) treatment of the elastic body layer can befacilitated, for example, so that the reliability can be furtherincreased.

According to a preferred embodiment of a method for manufacturing apiezoelectric device of the present invention, the pasting process ispreferably performed under reduced pressure. Due to the fact that thepasting process is preferably performed under reduced pressure, foamscan be suppressed in the vicinity of the interface of the elastic bodylayer, so that the reliability can be increased. Furthermore, since theheat treatment temperature can be made low, adverse effects, such as adeterioration of the cleavability or a deterioration of thecharacteristics of the piezoelectric substrate due to heat treatment canbe prevented.

According to a method for manufacturing a piezoelectric device of apreferred embodiment of the present invention, the inorganic layerformation process is preferably performed under reduced pressure. As aresult, foams (voids) in the vicinity of the interface of the inorganiclayer and the piezoelectric substrate can be prevented, so that a denseinterface can be formed.

According to various preferred embodiments of the present invention, theoccurrence of various problems during joining of the piezoelectricsubstrate with the sacrificial layer serving as the piezoelectric thinfilm in the piezoelectric device and the support member are preventedand the function as the piezoelectric device does not decrease. Thus, apiezoelectric device including a membrane structure in which the designdegree of freedom is higher than before, the process control is easy,and the characteristics and the reliability are excellent, can berealized.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a side cross sectional view illustrating theconfiguration of a piezoelectric device according to a first preferredembodiment of the present invention and a plan view as viewed from amounting surface side thereof.

FIG. 2 is a flow chart illustrating a method for manufacturing thepiezoelectric device according to the first preferred embodiment of thepresent invention.

FIGS. 3A-3E are views schematically illustrating manufacturing processesof the piezoelectric device formed according to a manufacturing flowillustrated in FIG. 2.

FIGS. 4A-4E are views schematically illustrating manufacturing processesof the piezoelectric device formed according to the manufacturing flowillustrated in FIG. 2.

FIGS. 5A and 5B are views schematically illustrating manufacturingprocesses of the piezoelectric device formed according to themanufacturing flow illustrated in FIG. 2.

FIG. 6 is a flow chart illustrating the method for manufacturing thepiezoelectric device according to the first preferred embodiment of theinvention.

FIGS. 7A-7C are views schematically illustrating manufacturing processesof the piezoelectric device formed according to a manufacturing flowchart illustrated in FIG. 6.

FIG. 8 is a side cross sectional view illustrating the configuration ofa piezoelectric device having another configuration of the invention.

FIGS. 9A and 9B are a side view illustrating the configuration of asurface acoustic wave device having the configuration of the inventionand a plan view as viewed from the mounting surface side thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A piezoelectric device according to a first preferred embodiment of thepresent invention and a method for manufacturing the piezoelectricdevice are described with reference to the drawings. In this preferredembodiment, the piezoelectric device is described taking a thin filmtype piezoelectric device for F-BAR containing a piezoelectric thin filmas an example.

FIG. 1A is a side cross sectional view illustrating the configuration ofthe piezoelectric device according to the present preferred embodiment.FIG. 1B is a plan view of the piezoelectric device as viewed from themounting surface side. In the plan view illustrated in FIG. 1B, theillustration of a solder resist 91 illustrated in FIG. 1A is omitted.FIG. 1A corresponds to the A-A′ cross section of FIG. 1B.

The piezoelectric device includes a piezoelectric thin film 10preferably having a thickness of about 1 μm made of a piezoelectricsingle crystal, such as LT, for example. For the piezoelectric thin film10, in addition to LT, materials which have piezoelectricity and can beseparated at a portion where the concentration of elements implantedinto a piezoelectric substrate reaches the peak, such as LN, LBO(Li₂B₄O₇), langasite (La₃Ga₅SiO₁₄), KN (KNbO₃), and KLN (K₃Li₂Nb₅O₁₅),may be acceptable.

On a front surface 13 of the piezoelectric thin film 10, an upperelectrode 50U, an upper drawing electrode 51LU, pad electrodes 51U, 51C,and 51UD are formed. The upper electrode 50U is formed in the shape of aflat plate having a certain area and is connected to the pad electrode51U through the upper drawing electrode 51LU. The pad electrode 51C isformed independently from the other electrodes. The pad electrode 51UDis connected to the pad electrode 51D at the side of a back surface 12through a via hole.

For the upper electrode 50U, Al, W, Mo, Ta, Hf, Cu, Pt, Ti, Au, or othersuitable material preferably are used alone or in combination inaccordance with the specification of the device, for example. Incontrast, Al, Cu, and other suitable material are preferably used forthe upper drawing electrode 51LU and the pad electrodes 51U, 51C, and51UD, for example.

Furthermore, on the front surface 13 of the piezoelectric thin film 10,a solder resist 91 is formed. In this case, the solder resist 91 ispreferably formed so as to exclude a region where a gap layer 60described later is formed when the piezoelectric thin film 10 is viewedin plan. Moreover, the solder resist 91 is formed in such a manner as toexclude the top of the pad electrodes 51U, 51C, and 51UD. On the padelectrode 51U, 51C, and 51UD, bumps are formed and a soldering ball 90is formed on each bump.

In contrast, on the back surface 12 of the piezoelectric thin film 10, alower electrode 50D, a lower drawing electrode 51LD, and a pad electrode51D are formed. The lower electrode 50D is formed so as to face theupper electrode 50U through the piezoelectric thin film 10. The lowerelectrode 50D is connected to the pad electrode 51D through the lowerdrawing electrode 51LD. For the lower electrode 50D, the same materialsas those of the upper electrode 50U are preferably used. For the lowerdrawing electrode 51LD and the pad electrode 51D, the same materials asthose of the upper drawing electrode 51LU and the pad electrodes 51U,51C, and 51UD are preferably used.

Furthermore, an inorganic layer 20 is disposed so as to abut on the backsurface 12 of the piezoelectric thin film 10, except for a region of acertain area including the formation region of the lower electrode 50Don the back surface 12 of the piezoelectric thin film 10. Morespecifically, the inorganic layer 20 is disposed at the back surface 12side of the piezoelectric thin film 10 so that a gap layer 60 is formedin a region of a certain area including the formation region of thelower electrode 50D. Used as materials of the inorganic layer 20 arematerials having an elastic modulus or hardness higher than a certainvalue under the use environment of general piezoelectric devices, e.g.,at about −55° C. to about +150° C. Specifically, various kinds of metaloxides, such as a silicon oxide, a silicon nitride, an aluminum oxide,an aluminum nitride, a tantalum oxide, DLC (Diamond like Carbon), amagnesium oxide, and an yttrium oxide, or glass materials, such as PSG,are used. For the inorganic layer 20, materials having high thermalconductivity to the piezoelectric thin film 10 or materials having a lowcoefficient of linear expansion may be used. The inorganic layer may beformed of a plurality of layers, e.g., a two-layer structure of a layerhaving a low coefficient of linear expansion and a layer having highthermal conductivity.

On the surface of the inorganic layer 20 opposite to the piezoelectricthin film 10, an elastic body layer 30 is entirely formed. As materialsof the elastic body layer 30, materials having a relatively low elasticmodulus and low hardness are preferably used. Specifically, materials,such as resin materials, e.g., epoxy resin, polyimide resin,benzocyclobutene resin, heterocyclic olefin resin, and a liquid crystalpolymer, are preferably used. For the elastic body layer 30, materialshaving high heat resistance or high chemical resistance may preferablybe used. Particularly in the case of a device used at a high temperatureof about 300° C. or higher, polyimide resin, benzocyclobutene resin, anda liquid crystal polymer are more preferably used. It is preferable alsofor the elastic body layer 30 to have high thermal conductivity.

By compounding an inorganic filler made of silica, alumina, or othersuitable material in the elastic body layer 30, the elastic modulus, thehardness, the thermal conductivity, and also the coefficient of linearexpansion can also be adjusted as appropriate.

To the surface of the elastic body layer 30 opposite to the inorganiclayer 20, a support member 40 is pasted. For the support 40, materialswhich have excellent processability and are inexpensive are used.Specifically, Si, glass, and ceramics such as alumina, are used.

By providing such a layer configuration, various kinds of action effectsdescribed below can be obtained.

Since the support member 40 and a composite layer of the piezoelectricthin film and the inorganic layer 20 are joined (pasted) to each otherthrough the elastic body layer 30, a level difference caused byirregularities and the like are absorbed by the elastic body layer 30,so that the generation of a local stress caused by the junction in thepiezoelectric thin film 10 can be prevented even when the flatnessdegree of each interface during junction is not high. In particular, inthe case of forming the membrane structure partially having the gaplayer 60 between the above-described piezoelectric thin film 10 and theinorganic layer 20, a process is required which includes forming asacrificial layer 70 made of a solid substance in a region serving asthe gap layer 60, and removing the sacrificial layer 70 from etchingwindows 80 formed in the piezoelectric thin film 10 after joining to thesupport member as described below. The front surface of the sacrificiallayer 70 is naturally projected from the back surface 12 of thepiezoelectric thin film 10 (more correctly, the piezoelectric singlecrystal substrate 1 during the formation of the sacrificial layer 70).Therefore, it is not easy to simultaneously achieve a high flatnessdegree in the back surface 12 of the piezoelectric single crystalsubstrate 1 and the front surface of the sacrificial layer 70, and alevel difference caused by irregularities and the like are more likelyto occur than in the case where the membrane structure is not adopted.However, when the elastic body layer 30 according to a preferredembodiment of the present invention is provided, the influence of such alevel difference is suppressed and the stress reducing action describedabove is obtained.

In this case, even when particles are present on the joined surface(pasted surface) of the support member 40 or the joined surface (pastedside) of the inorganic layer 20, a level difference caused by theparticles can also be absorbed by the elastic body layer 30, so that thegeneration of the stress can be prevented.

Furthermore, since the pressure applied to the piezoelectric singlecrystal substrate 1 during joining (pasting) is reduced by the elasticbody layer 30, the occurrence of chipping or the like can be suppressedeven in a state where the cleavability of the piezoelectric singlecrystal substrate 1 is high and ions are implanted.

Moreover, since the process control conditions can be relieved asdescribed above, the process control can be facilitated.

By disposing the support member 40 on the piezoelectric thin film 10through the elastic body layer 30, even when a difference in thecoefficient of linear expansion between the piezoelectric thin film 10and the support member 40 is high, a stress caused by the difference inthe coefficient of linear expansion is absorbed by the elastic bodylayer 30. Thus, a necessity of including the coefficient of linearexpansion in the selection conditions of the materials of the supportmember 40 is eliminated, so that the selectivity of the support member40 increases. As a result, inexpensive materials can be selected and thecost of the support member 40 which occupies the volume of the acousticwave device with a high ratio can also be reduced, so that an acousticwave device can be realized at a low cost. Moreover, since materialsexcellent in processability can also be used for the support member 40,the process load to the support member 40 can also be reduced.

When the elastic body layer 30 is arranged to directly abut on thepiezoelectric thin film 10, damping occurs as described above withrespect to conventional devices. However, by inserting the inorganiclayer 20 having an elastic modulus or hardness higher than that of theelastic body layer 30, the occurrence of damping can be prevented. Thus,a deterioration of the characteristics due to the structural factor bythe use of the elastic body layer 30 can be suppressed.

Thus, the use of the configuration of this preferred embodiment realizesa piezoelectric device having high reliability and excellentcharacteristics at a low cost.

Furthermore, by the use of a material having high thermal conductivityfor the inorganic layer 20, heat generated when the piezoelectric thinfilm 10 is driven is transmitted to the inorganic layer 20 to bedissipated. Therefore, power durability can be increased. Furthermore,by increasing also the thermal conductivity of the elastic body layer30, the heat transmitted to the inorganic layer 20 is more effectivelytransmitted to the elastic body layer 30 to be dissipated from theelastic body layer 30 and the support member 40, so that the powerdurability can be further increased.

By using a material having a low coefficient of linear expansion for theinorganic matter layer 20, the elongation of the piezoelectric thin film10 due to temperature changes or the like can be constrained by theinorganic layer 20, so that the temperature characteristics as anacoustic wave device can be improved.

By compounding an inorganic filler in the elastic body layer 30, theabove-described elastic modulus, hardness, thermal conductivity, orcoefficient of linear expansion can be determined as appropriate.Therefore, an acoustic wave device in accordance with the specificationof reliability or the specification of characteristics can be easilyrealized. For example, the conditions such that the coefficient oflinear expansion is about 20 ppm/° C. or lower, the thermal conductivityis about 0.5 W(m·K) or more, and the elastic modulus is about 1 GPa ormore which cannot be realized only by resin materials can also berealized while securing the adhesive strength during pasting byadjusting the volume ratio of the resin and the inorganic filler toabout 50:50 to about 10:90, for example.

Thus, further by determining the composition of the inorganic layer 20and the elastic body layer 30 as appropriate, a piezoelectric devicehaving much higher reliability and more excellent characteristics can berealized at a low cost.

Next, a method for manufacturing the above-described piezoelectricdevice is described with reference to the drawings.

FIG. 2 is a flow chart illustrating a method for manufacturing thepiezoelectric device of a preferred embodiment of the present invention.

FIGS. 3, 4, and 5 are views schematically illustrating manufacturingprocesses of the piezoelectric device formed by the method illustratingin the manufacturing flow chart illustrated in FIG. 2.

First, a piezoelectric single crystal substrate 1 having a certainthickness is prepared. Then, hydrogen ions are implanted from the backsurface 12 side as illustrated in FIG. 3A to thereby form an ionimplanted portion 100 (FIG. 2: S101). In this case, as the piezoelectricsingle crystal substrate 1, a substrate on which a plurality ofpiezoelectric device simple substances are arranged is preferably used.When an LT substrate is used for the piezoelectric single crystalsubstrate 1, for example, by implanting the hydrogen ions with a doseamount of about 1.0×10¹⁷ atom/cm² at an acceleration energy of about 150KeV, a hydrogen distributed portion is formed at a depth of about 1 μmfrom the back surface 12, so that the ion implanted portion 100 isformed. The ion implanted portion 100 is a portion where theconcentration of the ion elements implanted into the piezoelectricsingle crystal substrate reaches a peak. The ion implantation treatmentconditions are determined as appropriate in accordance with the materialof the piezoelectric single crystal substrate 1 and the thickness of theion implanted portion 100. When the acceleration energy is adjusted toabout 75 KeV, for example, a hydrogen distributed portion is formed at adepth of about 0.5 μm.

Next, as illustrated in FIG. 3B, the lower electrode 50D, the padelectrode 51D, and the drawing electrode 51LD, which is not illustrated,are formed on the back surface 12 of the piezoelectric single crystalsubstrate 1 (FIG. 2: S102).

Next, as illustrated in FIG. 3C, the sacrificial layer 70 is formed onthe back surface 12 of the piezoelectric single crystal substrate 1(FIG. 2: S103). Herein, the sacrificial layer 70 is formed in a regionhaving a certain area containing the lower electrode 50D, i.e., a regionserving as a main functional portion as an F-BAR element. Thesacrificial layer 70 is made of a material for which etching gas or anetching solution capable of differentiating the etching rate from thatof the lower electrode 50D can selected and contains a material which ismore easily etched than that of the lower electrode 50D. Moreover, thesacrificial layer 70 is made of a material which is more easily etchedthan the inorganic layer 20 described later or the piezoelectric singlecrystal substrate 1. It is more preferable for the sacrificial layer 70to contain a material having resistance to electromigration.Specifically, the material is determined as appropriate in accordancewith the conditions from metals, such as Ni, Cu, and, Al, insulationfilms, such as SiO₂, ZnO, PSG (phosphosilicate glass), organic films,and other suitable materials. The sacrificial layer 70 preferably islaminated and formed by vapor deposition, sputtering, CVD, or the likeor is formed by spin coating or the like.

Next, as illustrated in FIG. 3D, the inorganic layer 20 is formed on theback surface 12 of the piezoelectric single crystal substrate 1 with thesacrificial layer 70 (FIG. 2: S104). As materials of the inorganic layer20, materials satisfying the above-described elastic modulus, hardness,thermal conductivity, or coefficient of linear expansion are used andthe thickness is determined as appropriate.

As a method for forming the inorganic layer 20, a joining method is notused and a method is determined as appropriate in accordance with thespecification, manufacturing conditions, and the like from a CVD methodwhich is a directly forming method, a vapor deposition method, such as asputtering method and an E-B (electron beam) method, an ion platingmethod, a thermal spraying method, and a spray method. In this case, theinorganic layer 20 is formed at a temperature lower than the temperatureof a peeling process described later.

The inorganic layer 20 is preferably formed under reduced pressure.Thus, by forming the inorganic layer 20 under reduced pressure, thegeneration of voids (foams) in the interface of the back surface 12 ofthe piezoelectric single crystal substrate 1 and the inorganic layer 20is prevented and the like, so that the interface is densely formed.Thus, a highly reliable interface can be formed. Moreover, due to thefact the interface is stably and densely formed, a variation inreflection of acoustic waves on the interface or the like can besuppressed, so that the characteristics of an acoustic wave device canbe improved and the stabilization of the characteristics can also beincreased.

Next, as illustrated in FIG. 3E, the elastic body layer 30 is formed onthe surface of the inorganic layer 20 opposite to the piezoelectricsingle crystal substrate 1 (FIG. 2: S105). As materials of the elasticbody layer 30, materials satisfying an elastic modulus smaller than andhardness lower than those of the above-described inorganic layer 20 areused, and materials satisfying the above-described thermal conductivityand coefficient of linear expansion are more preferably used.

The formation method of the elastic body layer 30 is a coating method,for example. More specifically, as the coating method, the use of a spincoat method, a spray coat method, and a dispense method is morepreferable. In this case, the coating thickness is determined asappropriate in accordance with the characteristics required as theelastic body layer 30 and the elastic modulus peculiar to the materials.

Next, as illustrated in FIG. 4A, the support member 40 is pasted to thesurface of the elastic body layer 30 opposite to the inorganic layer 20(FIG. 2: S106). In this case, the pasting is performed under reducedpressure. Thus, by performing the pasting under reduced pressure, voidsin the pasted interface of the elastic body layer 30 and the supportmember 40 can be suppressed. Thus, a piezoelectric device having highreliability can be formed.

Next, as illustrated in FIG. 4B, a composite piezoelectric substratecontaining the piezoelectric single crystal substrate 1 on which theinorganic layer 20, the elastic body layer 30, and the support member 40are disposed is heated to thereby perform peeling using the ionimplanted portion 100 as the peeling surface (FIG. 2: S107). Thus, thepiezoelectric thin film 10 with the sacrificial layer 70 is formed andis supported by the support member 40 and on which the inorganic layer20 and the elastic body layer 30 are disposed. In this case, the heatingtemperature can be made low when heated under reduced pressure.

Next, the front surface 13 of the piezoelectric thin film 10 which ispeeling and formed as described above is polished by CMP treatment orthe like to be flattened so that the surface roughness Ra becomes lowerthan about 1 nm, for example. Thus, the characteristics of an acousticwave device can be improved. Then, polarization electrodes are formed onthe upper and lower surfaces of the composite piezoelectric substratecontaining the piezoelectric thin film 10, the inorganic layer 20, theelastic body layer 30, and the support member 40, and then a polingprocess is performed by applying a certain voltage to recover thepiezoelectricity of the piezoelectric thin film 10. In this case, thesacrificial layer may be utilized for the polarization electrodes as aconductive material.

Next, as illustrated in FIG. 4C, an upper electrode pattern, such as theupper electrode 50U which drives as an F-BAR device or the pad electrode51U, is formed on the front surface 13 of the piezoelectric thin film 10(FIG. 2: S108).

Next, as illustrated in FIG. 4D, the etching windows 80 for removing thesacrificial layer 70 and a through hole 81 for a via hole for conductionbetween the front surface 13 side and the back surface 12 side of thepiezoelectric thin film 10 are formed in the piezoelectric thin film 10.In this case, the etching windows 80 are formed in the vicinity of theend portions of the formation region of the sacrificial layer 70 in thepiezoelectric thin film 10. The through hole 81 is formed on the padelectrode 51D in the piezoelectric thin film 10. The etching windows 80and the through hole 81 are formed by dry etching utilizingphotolithography, for example. For the dry etching, NLD(Neural LoopDischarge)-RIE or SWP(Surface Wave Plasma)-RIE may be used, for example.

Next, as illustrated in FIG. 4E, a via hole electrode of the throughhole 81, the pad electrode 51UD connected thereto, the upper drawingelectrode 51LU connected to the pad electrode 51U, and the like areformed. The pad electrode 51C may be formed in this process or may beformed in the above-described process S108.

When passing through the processes so far, the composite is subjected toannealing treatment. By performing such annealing treatment, thecrystallinity of the piezoelectric thin film 10 which has sufferedcrystal damage in the ion implantation process can be recovered, andelongation or curvature of the piezoelectric thin film 10 is suppressed.Thus, even when the gap layer 60 is formed in the next removal processof the sacrificial layer 70, a fracture of a portion (piezoelectricmembrane portion) of the piezoelectric thin film 10 contacting the gaplayer 60 can be prevented.

Next, the sacrificial layer 70 is removed by causing etching gas or anetching solution flow through the etching windows 80. Thus, a spacewhere the sacrificial layer 70 is formed corresponding to the regionwhere the lower electrode 50D and the upper electrode 50U are formed ofthe piezoelectric device serves as the gap layer 60 as illustrated inFIG. 5A (FIG. 2: S109).

Next, as illustrated in FIG. 5B, the solder resist 91 is formed on thefront surface 13 of the piezoelectric thin film 10 except for the top ofeach of the pad electrodes 51U, 51UD, and 51C and the gap layer 60.Then, a bump is formed on each of the pad electrodes 51U, 51UD, and 51C,and soldering balls 90 are formed on the bumps. Thus, a piezoelectricdevice for F-BAR including the membrane structure is formed.

By the use of the manufacturing method described above, a piezoelectricdevice in which the piezoelectric thin film 10, the inorganic layer 20,the elastic body layer 30, and the support member 40 are laminated andwhich includes the membrane structure can be manufactured with highreliability and excellent characteristics. Furthermore, thepiezoelectric device can be manufactured without increasing the processload.

Next, a method for manufacturing a piezoelectric device according to yetanother preferred embodiment is described with reference to thedrawings. The piezoelectric device of the present preferred embodimenthas a unique feature included in the manufacturing method thereof. Theconfiguration as a final piezoelectric device is preferably the same asthe piezoelectric device of the first preferred embodiment, andtherefore the description of the configuration is omitted. Also in themanufacturing method, only characteristic portions are described and thedescription of the same processes as those of the first preferredembodiment is simplified.

FIG. 6 is a flow chart illustrating the method for manufacturing thepiezoelectric device of the present preferred embodiment.

FIGS. 7A-7E are views schematically illustrating characteristicprocesses different from the processes of the first preferred embodimentin the manufacturing process of the piezoelectric device formed by themethod illustrated in the manufacturing flow chart of FIG. 6.

First, as illustrated in FIG. 7A, the processes until the inorganiclayer 20 is formed on the piezoelectric single crystal substrate 1 withthe sacrificial layer 70 preferably are the same as those of the firstpreferred embodiment (FIG. 6: S201 to S204).

Separate from these processes, the elastic body layer is formed on thesurface of the support member 40 as illustrated in FIG. 7B (FIG. 6:S205). The material and the formation method of the elastic body layer30 are preferably the same as those of the first preferred embodiment.

Next, the support member 40 with the elastic body layer 30 is subjectedto baking treatment at a certain temperature (FIG. 6: S206). Forexample, in a case of manufacturing an acoustic wave device to be usedat about 300° C. or higher, the baking treatment is performed at atemperature in which a certain margin is added to the use conditiontemperature. By performing such a baking treatment, a solvent and thelike are emitted from the elastic body layer 30 during the bakingtreatment at a temperature equal to or higher than the temperature whichis not used in the manufacturing method of the first preferredembodiment and the use environment of the piezoelectric device, e.g.,under an environment of about 300° C. or higher. Thus, annealingtreatment is performed in accordance with a use situation, and theoccurrence of a deterioration of the characteristics of thepiezoelectric device during use can be prevented.

Herein, such a baking treatment cannot be carried out in themanufacturing method of the first preferred embodiment. This is becausewhen heat of about 300° C. or higher is applied to the compositepiezoelectric substrate containing the piezoelectric single crystalsubstrate 1, the inorganic layer 20, and the elastic body layer 30, thepiezoelectric single crystal substrate 1 is peeled at the ion implantedportion 100 to be formed into a thin film. However, the use of themanufacturing method of this preferred embodiment can prevent thepeeling and formation of the piezoelectric thin film 10 before thepeeling process.

Next, the elastic body layer 30 formed on the support member 40 and theinorganic layer 20 formed on the piezoelectric single crystal substrate1 with the sacrificial layer 70 are pasted to each other as illustratedin FIG. 7C (FIG. 6: S207). In this case, the surface of the inorganiclayer 20 is flattened. The process conditions of the pasting process maybe the same as those of the first preferred embodiment.

Then, the piezoelectric thin film is formed by heating and peeling (FIG.6: S208) and the formation of the upper electrode pattern is performed(FIG. 6: S209) preferably under the same conditions as those of thefirst preferred embodiment. Furthermore, preferably under the sameconditions as those of the first preferred embodiment, the sacrificiallayer 70 is removed to form the gap layer 60 (FIG. 6: S210), andfinally, a piezoelectric device having the shape illustrated in FIGS. 1Aand 1B is formed.

As described above, by the use of the manufacturing method of thepresent preferred embodiment, a piezoelectric device which is formedaccording to the process flow in which the temperature is about 300° C.or high can also be certainly manufactured with high reliability whilemaintaining excellent characteristics.

Although in the above-described preferred embodiments, an example ofusing a support member in the shape of a plate is described, a supportmember having a concave portion in a region corresponding to themembrane structure as illustrated in FIG. 8 may be used.

FIG. 8 is a side cross sectional view illustrating the configuration ofa piezoelectric device having another configuration according to anotherpreferred embodiment of the present invention. In the case of such aconfiguration, it is also realizable to make the thickness of theelastic body layer or the inorganic layer uniform.

Moreover, the above-described preferred embodiments describe apiezoelectric device for F-BAR that preferably includes the upperelectrode and the lower electrode facing each other on both surfaces ofthe piezoelectric thin film and having the membrane structure. However,the above-described configuration can be applied also to otherpiezoelectric devices having the membrane structure. For example, FIGS.9A and 9B are a side view illustrating the configuration of an acousticwave device of the invention and a plan view as viewed from the mountingsurface side thereof.

The acoustic wave device illustrated in FIGS. 9A and 9B preferably is anexample of an acoustic wave device using a Lamb wave, a plate wave, orthe like in which IDT electrodes 50P and drawing electrode 51L1 and 51L2are formed on the front surface 13 of the piezoelectric thin film 10 inplace of the upper electrodes 50U and 50D and the upper drawingelectrode 51LU of the piezoelectric device for F-BAR described above.Furthermore, in the acoustic wave device, the electrodes on the backsurface 12 side of the piezoelectric thin film 10 are not formed incontrast to the piezoelectric device for F-BAR. Even in the case of suchan acoustic wave device, the above-described configuration can beutilized and the same effects obtained in the above-describedpiezoelectric device for F-BAR and the manufacturing method thereof canbe obtained.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. The scope of the invention, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A piezoelectric device comprising: a driveelectrode; a piezoelectric thin film on which the drive electrode isprovided; and a support member disposed on one principal surface side ofthe piezoelectric thin film and including a membrane structure in whicha gap layer is provided between a region in which the drive electrode isdefined on the piezoelectric thin film and the support member; anelastic body layer between the piezoelectric thin film and the supportmember; and an inorganic layer between the piezoelectric thin film andthe support member, the inorganic layer being located at thepiezoelectric thin film side of the elastic body layer and including amaterial with a higher elastic modulus and a higher hardness as comparedwith the elastic body layer.
 2. The piezoelectric device according toclaim 1, wherein the elastic body layer includes an inorganic filler. 3.The piezoelectric device according to claim 1, wherein the inorganiclayer has a thermal conductivity higher than that of the piezoelectricthin film.
 4. The piezoelectric device according to claim 1, wherein theelastic body layer has a thermal conductivity higher than that of thepiezoelectric thin film and the inorganic layer.
 5. The piezoelectricdevice according to claim 1, wherein the inorganic layer has acoefficient of linear expansion lower than that of the piezoelectricthin film.
 6. The piezoelectric device according to claim 1, wherein thepiezoelectric device contains a material made of a Group 1 element.